The present invention relates to a waveguide type optical modulator/switch, which is applicable to various systems including those for high-speed optical communication, optical switching networks, optical information processing, optical image processing, etc.
The waveguide type optical modulator is the most important key element for realizing various systems including those for high-speed optical communication, optical switching networks, optical information processing, optical image processing, etc. Hitherto, waveguide type optical modulators have been fabricated with some interesting substrates by various methods of fabrication. Many of these optical waveguide type devices have a LiNbO.sub.3 substrate and a GaAs substrate. Internal diffusion of titanium in LiNbO.sub.3 provides for a convenient and comparatively simple method of producing low-loss strip waveguides. Important parameters of the waveguide type optical modulator are the driving power, modulation bandwidth and insertion loss. The bandwidth and driving power have a trade-off relation to each other. Research concerning the waveguide type optical modulator, has been centered on the optimization of the trade-off relation noted above.
The bandwidth of the waveguide type optical modulator is mainly dependent on the kind, material and shape of the electrode and the dielectric constant of the substrate. Traveling-wave electrodes are so often used for wide bandwidth applications. The concept of using traveling-wave electrodes is that the traveling-wave electrodes are made extensions of the driving transmission line. Thus, the traveling-wave electrode should have the characteristic impedance of the source and cable. The modulation speed in this case is restricted by the difference between the traveling time (i.e., phase-velocity or effective refractive index) with respect to light and microwave. There are two different traveling-wave electrode structures that can be used, i.e.:
1) an ASL (asymmetric strip line) or ACPS (asymmetric coplanar strip) electrode structure; and PA1 2) a CPW (coplanar waveguide) electrode structure. PA1 a) conductor loss (which is a function of the electrode material and parameters thereof); PA1 b) dielectric loss (which is a function of substrate characteristics); PA1 c) loss due to impedance mismatching with respect to 50 .OMEGA. source and load; PA1 d) loss due to higher order mode propagation (which is further increased where there are CPW electrodes); and PA1 e) connector loss. PA1 1) The thickness of the chip is as very small as the order of 0.1 mm. Therefore, holding, packaging and connection of fiber/fiber connector at end portions are difficult. This means that the total thickness of device at end portions thereof should be increased without increasing the thickness at the other portions of the device. PA1 2) The characteristic impedance is 43.5 .OMEGA., which is smaller than the optimum required value of 50 .OMEGA. for low-loss matching. Therefore, the microwave reflection in the device is increased to increase the total microwave loss in the device so as to reduce the attainable bandwidth. This means that it is necessary to obtain a structure having a characteristic impedance of 50 .OMEGA.. PA1 3) It is required to further reduce the conductor loss in the CPW electrode, thereby alleviating the microwave attenuation in the device for obtaining a high-speed (wide-band) optical modulator/switch.
In order to increase the bandwidth, the microwave effective refractive index n.sub.m should be reduced (from the value of 4.2) to be close to the light effective refractive index n.sub.o (typically 2.2 in the case of the LiNbO.sub.3 substrate).
The bandwidth of the traveling-wave modulator is restricted by the phase-velocity mismatch between microwaves and optical waves. This means that it is necessary to reduce the difference between the microwave and light effective refractive indexes by reducing the microwave effective refractive index. One way of reducing the microwave effective refractive index for increasing the bandwidth is based on the use of a thick electrode and a buffer layer. Modulators using ASL or ACPS electrode structures have already been proposed, as disclosed in a publication "33 GHzo cm Broadband Ti:LiNbO.sub.3 Mach-Zehnder Modulator", ECOC' 89, Research Treatise ThB22-5, pp. 443-446 (1989). According to this publication, the microwave effective refractive index is reduced by using a thick electrode layer (ASL or ACPS electrode structure) and a buffer layer. A problem in the ASL or ACPS electrode structure is that the bandwidth is restricted to around 12 GHz by microwave resonation due to chip cross section. In order to increase the bandwidth to above 12 GHz, the chip dimensions (both width and thickness) should be reduced to about 0.6 mm. The requirement that the thickness of the chip should be about 0.6 mm poses no significant problem. However, the requirement that the width of the chip should be about 0.6 mm, poses problems when the chip is held, mounted and packaged.
Another way of reducing the microwave effective refractive index is to use an air layer which is formed by using a metal shield in a conventional traveling-wave electrode structure. This is introduced in a research treatise "New Traveling-Wave Electrode Mach-Zehnder Optical Modulator with 20 GHz Bandwidth of and 4.7 V Driving Voltage at 1.52 .mu.m Wavelength", Electronics Letters, Vol. 25, No. 20, pp. 1,382-1,383 (1989). This structure has a problem that a special metal cover shielded plate having Grooves has to be produced with accurate dimensions. This requires special and difficult techniques, increases the steps of fabrication and reduces allowable manufacturing tolerances.
Besides, even if the phase-velocity mismatch between microwaves and optical waves can be alleviated by one of the above ways, there is a restriction imposed on the bandwidth of the modulator/switch by the microwave attenuation caused by the electrode structure. For example, even if perfect phase-velocity matching is obtained between microwaves and optical waves, the ultimate bandwidth of the device is small unless the microwave attenuation is reduced. Generally, the microwave attenuation in devices is caused by:
A novel structure for high-speed optical modulators is thus necessary, which has a characteristic impedance of about 50 .OMEGA., substantially perfect phase-velocity matching between microwaves and optical waves, small microwave attenuation, and further which readily permits a simple process of fabrication as an extension of the general process of electrode fabrication and not requiring any extra special shield.
Hitherto, there has been no wide bandwidth, low voltage Ti:LiNbO.sub.3 optical modulator using thick but conventional CPW electrodes. The inventor has realized such a wide bandwidth optical modulator using a thick but conventional CPW electrode structure, which solves some of the problems discussed above. This wide bandwidth optical modulator is introduced in a research treatise "A Wide-Band Ti:LiNbO.sub.3 Optical Modulator With A Conventional Coplanar Waveguide Type Electrodes", IEEE Photonics. Tech. Lett. Vol. 4, No. 9, pp. 1,020-1,022, 1992 (hereinafter referred to as a prior art example). According to this research treatise, by appropriately selecting the material and thickness of the buffer layer and those of the electrode, the microwave effective refractive index n.sub.m is reduced (from a value of 4.2) to be close to the light effective refractive index n.sub.o (typically 2.2 in case of the LiNbO.sub.3 substrate). Further, the inventor has reduced the microwave attenuation in the structure by alleviating loss due to higher order mode microwave propagation. This could be attained by reducing the thickness of the chip from 0.8 mm to about 0.1 mm. Consequently, a wide-band optical modulator could be realized.
Some of problems which remain without being solved in the above prior art example, are as follows: